Expectation for fuel cells has recently been raised as a measure coping with environmental issues. In particular, polymeric fuel cells using a proton conductive polyelectrolyte membrane have been promised for their capability of operating in low temperature and possibility of reduction in size and weight.
Very strongly acidic group-containing fluoropolymers exemplified by Nafion (registered trade name of Du Pont, hereinafter the same) are known as polyelectrolytes for polymeric fuel cells. However, the strongly acidic group-containing fluoropolymers, being fluorine-based polymers, are very expensive, and their synthesis and disposal call for consideration for the environment.
To address the expensiveness problem of the very strongly acidic group-containing fluoropolymers, a number of proposals have been made on polyelectrolyte membranes based on less expensive non-fluoropolymers. In particular, aromatic polyether sulfone type polymers are known from the standpoint of cost and durability of polyelectrolyte membranes.
With respect to applications of sulfonated polysulfone, sulfonated aromatic polyether sulfone, and aromatic polyaryl ether sulfone based polyelectrolytes to fuel cells, sulfonated homopolymer membranes, crosslinked membranes, polyblend membranes, inorganic acid blend membranes, and the like are disclosed in Nolte R. et al., J. Membr. Sci, vol. 83, 211 (1993), Note R. et al., BHR Group Conf Ser. Publ., vol. 3, 381 (1993), JP-T-8-509571 (corresponding to U.S. Pat. No. 5,733,678 and EP 698300), JP-A-10-21943, JP-A-11-116679, Kerres J. et al., J. Membr. Sci., vol. 139, 211 (1998), Yen S-P “E” et al., Proc. Power Sources Conf 38th, 469 (1998), Walker M. et al., J. Appl. Polym. Sci., vol. 74, 67 (1999), Baradie B. et al., Macromol. Symp., vol. 138, 85 (1999), Kerres J. et al., J. New Mater. ElectroChem. Systems, vol. 3, 229 (2000), Kerres J. A., J. Membr. Sci., vol. 185, 3 (2001), Genova-Dimitrova P. et al., J. Membr. Sci., vol. 185, 59 (2001), Stoler E. J. et al., Proceedings of 36th Intersociety Energy Conversion Engineering Conf, 975 (2001), Kim. Y. S. et al., Polymeric Mater: Sci. Eng., vol. 85, 520 (2001), and Wang F. et al., J. Membr. Sci., vol. 197, 231 (2002), etc.
JP-A-11-67224, etc. disclose a membrane electrode assembly using a sulfonated aromatic polyether sulfone polyelectrolyte membrane.
In particular, JP-A-11-116679 teaches a polyelectrolyte obtained by sulfonating a precursor polymer having a reduced viscosity of 0.6 to 1.5 dl/g.
JP-A-10-45913 (corresponding to U.S. Pat. No. 6,087,031) discloses a polyelectrolyte of sulfonated aromatic polyether sulfone having an ion exchange group equivalent weight of 800 to 5000 g/mol.
JP-A-11-67224 discloses an electrolyte of a sulfonated aromatic polyaryl ether sulfone.
JP-A-10-21943 (corresponding to U.S. Pat. No. 5,985,477 and EP 932213) discloses a polyelectrolyte of a sulfonated aromatic polyether sulfone copolymer having an ion exchange group equivalent weight of 500 to 2500 g/mol.
However, concrete description in these publications is confined to random copolymers or blends of homopolymers. There is no mention of an aromatic polyether sulfone block copolymer composed of a hydrophilic segment with a sulfonic acid group and a hydrophobic segment with no sulfonic acid group nor reference to humidity dependence of proton conductivity.
In a solid polymeric fuel cell, fuel (ordinarily hydrogen) is usually humidified to supply moisture to the polyelectrolyte membrane so that the polyelectrolyte membrane is used with an absorbed water content. Therefore, it is desirable for the polyelectrolyte membrane to have unchanged proton conductivity with variation of the humidification degree of the fuel. The heretofore proposed sulfonated polysulfone, sulfonated aromatic polyether sulfone, and sulfonated aromatic polyaryl ether sulfone based polyelectrolyte membranes are disadvantageous in that their ion conductivity varies largely with moisture content (humidity) or temperature of the fuel supplied.
JP-A-2-294338 discloses a maltilayer ion-exchange membrane comprising a sulfonated aromatic polyether sulfone block copolymer but has no mention of the relation between proton conductivity and humidity or temperature variations.
JP-A-2001-278978 (corresponding to U.S. Patent Application 20010021764) discloses a block copolymer comprising sulfonated aromatic polyether sulfone blocks and unsulfonated aromatic polyether sulfone blocks but has no mention of application as an electrolyte membrane for fuel cells, still less humidity or temperature dependence of proton conductivity.
JP-A-2001-250567 (corresponding to U.S. Patent Application 20010041279 and EP 1113517) discloses a block copolymer comprising a block containing sulfonic acid groups and a block containing no sulfonic acid groups for use as a polyelectrolyte membrane of a fuel cell. The publication teaches that the block copolymer has a suppressed water absorption and therefore exhibits excellent water resistance while being equal or superior in ion conductivity to polyelectrolytes having sulfonic acid groups introduced randomly. However, no mention is made of humidity dependence of proton conductivity. It is difficult to anticipate a solution to the above-described problem from the disclosure. Further, JP-A-2001-250567 describes that the block copolymer preferably contains 60% by weight or more of the blocks having no sulfonic acid groups based on the whole copolymer and that a proportion smaller than 60% can result in reduction of water resistance. Comparing among polyelectrolytes of the same structure, however, a higher proportion of blocks having sulfonic acid groups introduced generally leads to higher proton conductivity as is preferred. Polysulfone, polyether ether sulfone, etc. are mentioned as the sulfonic acid group-containing blocks but not as preferred structures. There is no mention of use of aromatic polyaryl ether sulfone as a sulfonic acid group-containing block. Many of the specific examples given are blocks of epoxy resins. These blocks, having an aliphatic main chain, cause a reduction in heat resistance.